1. Field of the Invention
This invention relates to digital-to-digital code converters; and it relates, in particular to a circuit for converting a digital time-amplitude code to a digital amplitude code.
2. Description of the Prior Art
A copending application of R. C. Brainard and J. C. Candy, Ser. No. 461,878, filed Apr. 18, 1974, teaches an encoding and decoding technique in which information is represented in a time-amplitude format. That is, magnitude information is represented in part in a so-called shift-companded, or n:m, code format and inpart in the values assumed in that format over recurrent intervals of time. In that format a word of w bits includes n most significant bit ZEROs followed in positions of decreasing binary significance by m least significant bit ONEs. Thus, the designation n:m code is hereinafter usually employed. This code format is conveniently generated by using a difference coded signal, such as delta-modulation type signal pulse train bits, to determine the direction of shifting of a shift register which is driven at the bit rate of the pulse train. The shift register is so wired that binary ONEs are injected in the least significant bit stage when the register shifts toward the most significant bit stage, and binary ZEROs are injected in the most significant bit stage when the register shifts toward the least significant bit stage -- hence the concept of a shift-companded code.
Each word of the n:m code represents a segment, or amplitude range, of a segmented, or linear piecewise approximation, pulse code such as a .mu.-law companded digital pulse code. The Brainard et al. coder circuit parts which restore the n:m code to analog form for feedback purposes are biased so that each code word actually represents an analog amplitude that is so chosen within the corresponding .mu.-law code segment that the average value of any two successive n:m code words is equal to the value, in terms of numbers of companded code unit-segment-sized amplitude units, of the intermediate .mu.-law segment boundary. Thus, it should be apparent that an n:m code word provides only a coarse amplitude representation. Greater resolution arises from the time dimension in the way that the code words are used. Each n:m code word corresponds to a different bit time of the differential pulse code train; and it has been found that, if the reconstituted analog step information can be averaged over a Nyquist period of the underlying analog information, the average will be substantially equal to the average magnitude of the underlying analog signal sample magnitude in that same period. The use of the n:m code thus greatly facilitates analog-to-digital and digital-to-analog conversions. However, that code is not convenient for direct utilization in commercial transmission systems which often employ digital words in a .mu.-law companded code format. Similarly, the n:m code is not convenient for data processing which often employs the linear 2s-complement pulse code modulation format.
Many digital-to-digital code converting techniques are known in the art for achieving conversion without first decoding the received signal to analog format prior to recoding it in the new digital format. The advantages of such digital conversions are well known. However, those prior techniques usually deal with amplitude-representative codes with one character per Nyquist interval for fully representing the signal amplitude during that interval. Such prior techniques are not useful for time-amplitude-representative codes such as the aforementioned n:m code in which plural characters in a Nyquist interval each provides a coarse amplitude representation that is refined by considering as a group the characters of that time interval. Although there are known code conversion techniques for translating delta modulation signals to some other format by direct accumulation of the increment and decrement information in the delta modulation pulse train, these methods are unsuitable for the n:m code in which each word is itself already an accumulation and not an increment. Furthermore, in the n:m code, the code words are only a coarse representation of analog information, which representation does not directly accumulate accurately; whereas, the delta form is a fine-grained incremental representation which does directly accumulate accurately. In addition, the delta modulation form is usually a linear representation which can be conveniently accumulated and not n:m companded form which cannot be conveniently directly accumulated.